Currently, many full-service DTV stations perform both linear and non-linear distortion correction while the transmitter is “on-line” (i.e., while the desired DTV signal is being transmitted over-the-air). Distortion may be caused, for example, by certain elements, such as the power amplifier, of the transmitter of a DTV station. A separate, accurate, and costly reference receiver is often used to accurately measure this distortion. Correction information based on the measured distortion is then loaded into the DTV modulator to pre-distort the VSB signal. Accordingly, the pre-distortion of the VSB signal negates the distortion caused by the transmitter so as to provide a clean output signal to the transmit antenna. However, cost prohibits the use of this reference receiver in inexpensive DTV translators.
Digital 8-VSB has been the U.S. standard for digital television transmission since December 1996 when the FCC selected the ATSC standard (minus table 3) as the 6 MHz digital transmission standard for this country. Likewise, MPEG-2 video coding and Dolby's AC-3 audio coding were also selected as part of the standard along with MPEG transport data stream protocols. Volunteer DTV stations began transmitting digital 8-VSB in November of 1998, with the official start for commercial DTV stations beginning in May 1999. The VSB transmission system is described in the document entitled “ATSC Digital Television Standard—A/53B”, and can be found on the ATSC website (www.atsc.org).
Full service DTV stations in the U.S. typically meet the ATSC recommended compliance factors as specified in ATSC document A/64A. These factors include recommendations for both in-band signal quality as well as adjacent channel splatter interference. The in-band signal quality is often described in terms of signal-to-noise ratio/modulation-error-ratio (SNR/MER), which describes the “openness” of the data eyes in the presence of the transmitter's linear and non-linear distortion (white Gaussian thermal noise in a transmitter is not a consideration). Typically, these full service stations have completely separate test equipment to not only measure the signal quality for compliance, but also to correct the linear and non-linear distortion in the transmitter by pre-distorting the signal in the VSB modulator. This test equipment is typically of instrument-grade quality and very expensive.
FIG. 1 illustrates a typical full service DTV station 10, with an MPEG transport stream as an input and a VSB-modulated RF signal as an output. The DTV station 10 includes a transmitter 12 having a VSB modulator 14, an RF upconverter 16, and a high power amplifier 18. The VSB modulator 14 modulates the MPEG transport stream as a VSB signal, usually an 8 VSB signal. The output of the VSB modulator 14 is upconverted by the RF upconverter 16 to the frequency corresponding to the output channel of the DTV station 10, and the upconverted signal is amplified by the high power amplifier 18. The DTV station 10 also includes an emission mask filter 20 that is typically a narrow bandpass filter to confine the signal to be up linked by a device such as an antenna 22 to the correct channel bandwidth. As suggested previously, the high power amplifier 18 and/or the emission mask filter 20 distorts the signal to be up linked by the antenna 22.
Therefore, the DTV station 10 also includes a reference VSB receiver 24. The reference VSB receiver 24 is shown as an external piece of equipment, separate from the transmitter 12 itself, so that the broadcaster can independently and accurately measure the signal to be transmitted. To this end, the reference VSB receiver 24 receives the signal to be transmitted by way of a directional coupler 26, measures the received signal, and supplies correction data to the taps of a correction filter in the transmitter 12 to compensate for linear distortion imposed by the high power amplifier 18 and/or the emission mask filter 20. The reference VSB receiver 24 operates “on-line”, that is, while the DTV signal is being transmitted by the DTV station 10, and without service interruption.
There are some manufacturers who place a separate reference receiver inside their transmitters. However, correction is typically not obtained with the same accuracy as that obtained from an external piece of instrument-grade test equipment.
The cost, in either case, is not insignificant. However, since full service DTV stations buy very expensive transmitters, this cost is not prohibitive for many of those stations.
DTV translators, on the other hand, are very small and inexpensive receiver/transmitter units placed at key locations in or beyond the service area of full service DTV stations to provide better signal levels and signal quality to terrain shielded areas. They are often placed on “mountain top” sites, providing signals to terrain shielded valleys.
The primary purposes of DTV translators are to essentially relay the MPEG transport data unprocessed (except for a few minor PSIP changes in the transport stream for identification purposes), and to translate the incoming DTV signal to a new channel frequency. There are two types of DTV translators: heterodyne and regenerative. The heterodyne type of DTV translator just mixes the incoming RF signal on RF channel A down to an IF frequency, bandpass filters it, and then upconverts it to the new RF channel B. No equalization (ghost canceling) is performed, nor is any error correction performed on the data. Therefore, the signal quality is not improved, but rather is the same or worse at its output.
The regenerative type of DTV translator actually decodes the VSB signal, performs both equalization and error correction, and then re-modulates and upconverts the MPEG-transport data to the new RF channel. More specifically, the regenerative type of DTV translator provides the following functions: select the desired DTV signal on the appropriate incoming RF channel, rejecting any interference signals (Tuner/Demod); remove any multipath from the signal (Equalizer); perform error correction and decoding to obtain an error-free transport data stream (VSB Decoder); perform simple PSIP data processing on the transport stream and regenerate the data clock (Data/Clock Processor); encode and re-modulate the data as a “pristine” 8T-VSB modulated signal at IF (VSB Modulator); upconvert the modulated signal to a new DTV channel frequency (Upconverter); and, amplify and (emission mask) filter the final output signal for transmission (High Power Amp/Mask Filter).
A typical regenerative DTV translator 30 is shown in FIG. 2, which contains a receive antenna 32, a VSB transcoder 34, a high power amplifier (HPA) and emission mask filter 36, and a transmit antenna 38. The VSB transcoder 34 is made up of two parts: a VSB receiver 40 and a VSB transmitter 42. The VSB receiver 40 includes a tuner and VSB demodulator 44, an equalizer 46, and a VSB decoder 48. The VSB receiver 40 is essentially identical to that found in consumer DTV sets, and provides an error-free MPEG transport data stream if the RF input signal is above the threshold of errors. The tuner and VSB demodulator 44 converts the RF signal first to IF and then demodulates the IF signal to baseband, rejecting any adjacent channel interfering analog NTSC or digital ATSC signals on other channels. The equalizer 46 specifically removes multipath (“ghosting”) effects that occur in propagation, and is typically configured as a combination of a finite-impulse-response (FIR) filter and a decision-feedback equalizer (DFE) circuit, which is essentially an infinite-impulse-response (IIR) filter. The VSB decoder 48 performs decoding, de-interleaving, de-randomization, and error-correction, providing an error-free transport data stream for VSB re-transmission. The MPEG transport data stream can also be used locally for decoding to baseband video and audio, if needed, at the translator site for NTSC re-transmission during the transition years.
The VSB transmitter 42 includes a data and clock processor 50, a pre-equalizer 52, and a VSB modulator and upconverter 54. The VSB transmitter 42 is similar to that found in a full-service commercial DTV transmitter. The data and clock processor 50 manipulates a few data packets in the transport streams (such as the “virtual channel” and “short name”) and locks the transmitter symbol clock to the incoming symbol clock (which can clean up any existing clock jitter present from the source full-service transmitter or created during propagation). The optional (short) pre-equalizer 52 is typically a FIR filter that corrects for any linear distortion in the output of the transmitter's high power amplifier and emission mask filter 36. As an example, this tapped-delay-line FIR filter with variable gain coefficients may contain 32 taps. The VSB modulator and upconverter 54 performs the VSB coding (randomization, interleaving, Reed-Solomon coding, trellis-coded modulation) and creates the pristine 6 MHz bandwidth IF data spectrum before being upconverted to the desired RF channel. The taps of the pre-equalizer 52 can be set by an external reference receiver such as the reference VSB receiver 24 shown in FIG. 1.
A controller (not shown) controls the typical functions, such as tuning, of the VSB transcoder 34.
There are two types of signal distortion that the RF output signal of the DTV translator may contain: linear and non-linear. While full service DTV station transmitters often remove both of these types of distortion automatically, the technology does not exist to remove this distortion automatically in a low-cost DTV translator. If needed, some of the non-linear distortion may be removed manually in the low-level analog IF circuits of the translator's exciter.
Since regenerative translators actually decode and error-correct the baseband transport data stream, the primary source of linear distortion in the output of the DTV translator is typically the narrow emission mask bandpass filter, whose main purpose is to remove the amplifier's adjacent channel splatter energy caused by any non-linearities. However, translator equipment cost prohibits the use of an expensive external reference receiver such as that shown in FIG. 1 to determine the correction taps needed to pre-distort the signal and essentially pre-equalize the entire translator for linear distortion.
The present invention is directed to performing correction in a low cost DTV translator. If desired, this correction can be done using essentially the same components as found in typical regenerative DTV translators, with only a handful of additional miscellaneous passive components.